Clean, efficient metal electrolysis via som anodes

ABSTRACT

In some aspects, the invention relates to apparatuses for recovering a metal comprising providing a sealed container for holding a molten electrolyte, the container having an interior surface; a liner disposed along at least a portion of the interior container surface; a cathode disposed to be in electrical contact with the molten electrolyte when the molten electrolyte is disposed in the container; a solid oxygen ion-conducting membrane disposed to be in ion-conducting contact with the electrolyte when the molten electrolyte is disposed in the container; an anode in contact with the solid oxygen ion-conducting membrane, the solid oxygen ion-conducting membrane electrically separating the anode from the molten electrolyte; and a power source for generating an electric potential between the anode and the cathode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. provisionalpatent application Ser. No. 61/843,798, filed Jul. 8, 2013, entitled“Clean, Efficient Aluminum Electrolysis via SOM Anodes”, the disclosureof which is hereby incorporated by reference in its entirety for allpurposes.

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Thepatent and scientific literature referred to herein establishesknowledge that is available to those skilled in the art. The issuedpatents, applications, and other publications that are cited herein arehereby incorporated by reference to the same extent as if each wasspecifically and individually indicated to be incorporated by reference.In the case of inconsistencies, the present disclosure will prevail.

GOVERNMENT SUPPORT

This invention was made with government support under Grant No. 1026639awarded by the National Science Foundation and Award No. DE-AR0000412awarded by the Department of Energy. The government has certain rightsin the invention.

FIELD OF THE INVENTION

The invention relates to apparatuses and methods for production ofmetals from metal oxides.

BACKGROUND OF THE INVENTION

The Hall-Héroult cell revolutionized aluminum production in 1886 (U.S.Pat. No. 400,664; herein incorporated by reference in its entirety) byreducing aluminum oxide dissolved in a molten salt, with a consumablecarbon anode that reacts with the oxygen to form carbon dioxide.However, the Hall-Héroult molten salt electrolysis remains limited incost and energy efficiency for several reasons. Production of therequired carbon anode is costly, as well as carbon- andenergy-intensive. In addition, liquid cryolite is extremely corrosive,such that containment thereof in air requires a “frozen sidewall” ofcryolite, which rapidly removes thermal energy from the cell. Moreover,emission of hot carbon dioxide from the anode contains HF, which rendersheat recovery from exiting gases impractical, and perfluorocarbons,powerful greenhouse gases with global warming potential (GWP) of6000-9000 times that of CO₂.

Use of a solid electrolyte, such as stabilized zirconia, between themolten salt and anode removes the anode requirement of chemicalstability in contact with a molten salt. For reactive metals such asaluminum, magnesium, calcium, sodium, potassium and rare earth metals,the solid electrolyte improves current efficiency considerably bypresenting a solid barrier between the metal produced at the cathode andoxidizing gases produced at the anode, preventing back-reaction (see,for example, U.S. Pat. Nos. 5,976,345 and 6,299,742; each hereinincorporated by reference in its entirety). The process comprises asolid oxygen ion-conducting membrane (SOM) typically consisting ofzirconia stabilized by yttria (YSZ) or other low valenceoxide-stabilized zirconia, for example, magnesia- or calcia-stabilizedzirconia (MSZ or CSZ, respectively) in contact with the molten saltelectrolyte bath in which the metal oxide is dissolved, an anode inion-conducting contact with the solid oxygen ion-conducting membrane,and a power supply for establishing a potential between the cathode andanode.

Due to the SOM process, consumable carbon anodes are not required andvolatile contamination of anode gases is mitigated. However, heat lossand crust formation remain an issue. Thus, there remains a need for moreefficient methods and systems to produce metals. This inventionaddresses these needs.

BRIEF SUMMARY OF THE INVENTION

In one aspect, an apparatus for recovering a metal is providedcomprising: (a) a sealed container for holding a molten electrolyte, thecontainer having an interior surface; (b) a liner disposed along atleast a portion of the interior container surface; (c) a cathodedisposed to be in electrical contact with the molten electrolyte whenthe molten electrolyte is disposed in the container; (d) a solid oxygenion-conducting membrane disposed to be in ion-conducting contact withthe electrolyte when the molten electrolyte is disposed in thecontainer; (e) an anode in contact with the solid oxygen ion-conductingmembrane, the solid oxygen ion-conducting membrane electricallyseparating the anode from the molten electrolyte; and (f) a power sourcefor generating an electric potential between the anode and the cathode.

In another aspect, a method for recovering a metal is providedcomprising: (a) providing a sealed container for holding a moltenelectrolyte, the container having an interior surface; (b) providing aliner disposed along at least a portion of the interior containersurface; (c) providing a cathode disposed to be in electrical contactwith the molten electrolyte when the molten electrolyte is disposed inthe container; (d) providing a solid oxygen ion-conducting membranedisposed to be in ion-conducting contact with the electrolyte when themolten electrolyte is disposed in the container; (e) providing an anodein contact with the solid oxygen ion-conducting membrane, the solidoxygen ion-conducting membrane electrically separating the anode fromthe molten electrolyte; (f) dissolving at least a portion of an oxide ofthe metal into the electrolyte; (g) establishing a non-oxidizingenvironment within the container; and (h) generating an electricpotential between the anode and the cathode, whereby the oxide of themetal is reduced to form metal.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are illustrative only and are not intended to belimiting.

FIG. 1. An illustrative embodiment of an electrolytic cell configurationand method according to an embodiment of the invention.

FIG. 2. An illustrative embodiment of an electrolytic cell configurationand method according to an embodiment of the invention.

FIG. 3. An illustrative embodiment of an electrolytic cell configurationand method according to an embodiment of the invention.

FIG. 4. An illustrative embodiment of an electrolytic cell configurationand method according to an embodiment of the invention.

DETAILED DESCRIPTION

As used herein and in the appended claims, the singular forms “a,” “an,”and “the” include plural references unless the content clearly dictatesotherwise.

The term “about” is used herein to mean approximately, in the region of,roughly, or around. When the term “about” is used in conjunction with anumerical range, it modifies that range by extending the boundariesabove and below the numerical values set forth. The term “about” is usedherein to modify a numerical value above and below the stated value by avariance of 20%.

The Hall-Héroult process electrical energy efficiency is below 55%(“U.S. Energy Requirements for Aluminum Production,” U.S. Department ofEnergy, EERE Report, February 2007; herein incorporated by reference inits entirety), and generates about 0.9 kg CO₂-equivalent (CO₂e) directgreenhouse gas (GHG) emissions per kg of aluminum produced, leaving muchroom for improvement. Its molten cryolite salt bath loses heat rapidlythrough the frozen cryolite cell “side-wall”. HF in off-gases makes heatexchanger energy recovery impossible, let alone capture for by-productsale or sequestration. Moreover, production of carbon anodes addsconsiderable energy use and expense. The industry has spent the past 125years trying new innovations.

The aluminum industry has tried various approaches to non-consumableinert anodes to avoid the high cost of carbon anodes. Approaches haveincluded cermets, generally of doped nickel ferrites with base/noblemetal alloys, transition metals which form conducting surface oxides,composites which combine these two, aluminum bronzes generally withcopper, and porous graphite with fuel gas flow to the outer surface.See, e.g., U.S. Pat. Nos. 4,397,729; 4,960,494; 5,254,232; 5,865,980;6,030,518; 6,113,758; 6,126,799; 6,217,739; 6,248,227; 6,284,562;6,322,969; 6,372,009; 6,372,119; 6,416,649; 6,423,195; 6,423,204;6,521,115; 6,758,991; 6,821,312; 6,878,247; 7,033,469; 7,235,161;7,431,812; 7,507,322; and 8,366,891; and Sankar Namboothiri, Mark P.Taylor, John J. J. Chen, Margaret M. Hyland and Mark Cooksey, “Aluminumproduction options with a focus on the use of a hydrogen anode: areview,” Asia-Pacific Journal of Chemical Engineering 2(5):442-447(2007); each herein incorporated by reference in its entirety.

Aluminum trichloride electrolysis with non-consumable multipolar carbonelectrodes as also been attempted. Producing anhydrous AlCl₃ without HClevolution, and operating a chlorine compressor, add considerably morecost than the savings in electrical energy and anode production.Aluminum trichloride is also acutely toxic, with health hazard 3 out ofa maximum of 4 on the NFPA diamond scale (See, e.g.,

http://www.sciencelab.com/msds.php?msdsId=9922851, herein incorporatedby reference in its entirety). This approach has never proveneconomically viable.

Low-temperature aluminum trichloride electrolysis with ionic liquidsloses less energy than a molten salt cell; however anhydrous AlCl₃production cost and toxicity make this unworkable.

Drained cathodes such as slanted TiB₂ plate cathodes drain off liquidaluminum into wells as it is produced. This reduces anode-cathodedistance (ACD) and resistance heat generation, as well asmagneto-hydrodynamics (MHD) interactions of the high magnetic field withthe otherwise thick liquid aluminum pad. The slant also improvesproductivity per unit area. However, the carbon anode, frozen sidewall,and anode gas issues remain.

Today's aluminum plants are modular, but must locate near low costelectric power sources such as dams. Carbothermic reduction plants maypotentially have lower energy cost location flexibility to provideliquid metal at customer facilities. However, carbothermic reduction ofaluminum requires very high temperature well above 2100° C., leading tosignificant materials and energy management challenges.

With the exception of carbothermic reduction, all of the above canretrofit into existing plants. However, for the reasons mentioned above,none of these technologies is in widespread use.

Development of the solid oxide membrane (SOM) electrolysis process hasprovided an alternative method for refinement of metal oxides (see, e.g,U.S. Pat. Nos. 5,976,345, and 6,299,742; each herein incorporated byreference in its entirety). The process as applied to metal productionconsists of a metal cathode, a molten salt electrolyte bath thatdissolves the metal oxide that is in electrical contact with thecathode, a solid electrolyte oxygen ion conducting membrane (SOM)typically consisting of zirconia stabilized by yttria (YSZ) or other lowvalence oxide-stabilized zirconia, for example, magnesia- orcalcia-stabilized zirconia (MSZ or CSZ, respectively) in ion-conductingcontact with the molten salt bath, an anode in ion-conducting contactwith the SOM, and a power source for establishing a potential betweenthe cathode and anode. Metal cations are reduced to metal at thecathode, and oxygen ions migrate through the membrane to the anode wherethey are oxidized to produce oxygen gas or other oxides. The SOM blocksback-reaction between anode and cathode products. It also blocks ioncycling, which is the tendency for subvalent cations to be re-oxidizedat the anode, by removing the electronic connection between the anodeand the metal ion containing molten salt because the SOM conducts onlyoxide ions, not electrons (see, U.S. Pat. Nos. 5,976,345, and 6,299,742;each herein incorporated by reference in its entirety); however theprocess runs at high temperatures, typically 1000-1300° C. in order tomaintain high ionic conductivity of the SOM.

The SOM electrolysis can proceed with high- and low-densityelectrolytes, and in some embodiments combines several desiredcharacteristics of metal oxide reduction technology including, forexample, 1) low-cost, low-toxicity metal oxide feedstock to avoid costsof producing, dehydrating and handling toxic metal chlorides; 2)insulated molten salt containment to reduce heat loss from the frozensidewall; 3) separation between anode gas and molten salt for reducedemission of HF contaminated gases and heat recovery from the voluminousanode gas; 4) low anode-cathode distance for low resistance and excessheat production; 5) scale-down ability to mini-mills and on-site sale ofliquid metal to customers; 6) ability to retrofit existing metalsmelters; 7) use of clean, inexpensive fuel (e.g., natural gas) withhigh energy content and half the CHG emissions of expensive carbonanodes, optionally with uncontaminated carbon dioxide capture forsequestration or sale as a by-product; and 8) optional oxygen-producinginert anode without contamination of the oxygen by-product by HF,fluorine or other toxic gases. Moreover, anodes that switch betweenfueled and inert operation can take advantage of fluctuating electricitycosts due to intermittent renewable energy sources or low demand periods(such as overnight).

The zirconia solid electrolyte represents a fundamental departure fromother molten salt electrolysis anodes and is advantageous over othermolten salt electrolysis anodes for several reasons including, e.g.,blocking back-reaction between dissolved metal in salt and oxygen orcarbon dioxide anode product, presenting a solid barrier that separatesthe anode gases from the molten salt, prevents the anode effect(eliminating periodic outages and carbon tetrafluoride emissions),enables multiple inert anode materials such as perovskites and liquidmetals, prevents carbon contamination of the product, and blocks otheranions, resulting in high purity oxygen by-product or combustionoxidant.

In addition, advantages particular to primary production of metal, e.g.,aluminum, include, e.g., eliminating carbon anode costs, clean emissionswith no anode effect from SOM selectivity, anode flexibility and energyefficiency, reduction of thermal loss, simpler metal collection,simplified feeding and improved electrode geometry.

Eliminating Carbon Anode Costs. The consumption of carbon anodes intraditional methods is not only expensive, but also requires anadditional production line located next to most metal smelteroperations. Its operation involves handling carcinogenic materials thatcan contaminate ground water. Eliminating the anode plant willsignificantly reduce capital expenditures, operating cost, energy useand several types of emissions.

Clean Emissions and No Anode Effect from SOM Selectivity. SOMselectivity delivers anode product gas with no HF, perflurocarbons (noanode effect) and no fluorine or oxyfluorides, thus making it suitablefor gas heat exchange and raw material preheating. Using this heat forpartial aluminum hydroxide calcining (>180-200° C.; Hollingbery, L. A.and Hull, T Richard, “The Structure and Thermal Decomposition ofHydromagnesite and Huntite—A review,” Thermochimica Acta 509:1-11(2010); herein incorporated by reference in its entirety) can reduceupstream energy use even further. Avoided costs include, e.g., noemissions control expense or energy use and/or lower permitting costsand delays.

Anode Flexibility and Energy Efficiency. The zirconia solid electrolytetubes can accommodate either liquid silver or perovskite inert anodesthat produce pure oxygen by-product or fueled (e.g., natural gas) anodesemitting only water and carbon dioxide. Hot swapping anode assembliesare also contemplated, as is exchanging current collectors while leavingzirconia tubes in place to change between inert and fueled (e.g.,natural gas) operation for GHG and cost savings.

Reducing Thermal Loss. The molten salt (cryolite) bath used in theHall-Heroult cell requires a “frozen sidewall” of cryolite forcontainment since it dissolves all refractory oxides and acceleratesoxidation of most metals. This frozen sidewall withdraws an enormousamount of thermal energy from the process, which increases energyconsumption dramatically. With SOM technology, oxygen originates insidethe SOM tube(s), and the bath produces little to no gas. Thus, a metalcontainer can contain the salts without a frozen side wall, there is nooxidant (e.g., oxygen, carbon dioxide or water) outside of the tube(s)that would oxidize the metal container, and anode gas originates in theSOM and does not pick up corrosive volatiles (e.g., HF) so it is easy toseal and inert the container using very little inert gas, e.g., argon ornitrogen, preventing oxidation of a metal container. Such aconfiguration permits advantageous operation in a sealed, e.g., welded,steel vessel surrounded by thermal insulation, eliminating much of thethermal losses.

Improved Electrode Geometry. Vertical plate or rod cathodes with rows ofzirconia anode assemblies between them result in low anode-cathodedistance similar to drained cathodes, and provide much more electrodesurface area per unit cross-section area for a given cell size. Thisboth reduces energy losses and reduces building size for a givenproduction volume.

Simpler Metal Collection. When the molten salt is denser than the metal,the metal floats on top of the cell. This enables tapping of the cell ata side collection well. The new cell geometry is without a bottom entry,making it less likely to leak.

No MHD-induced Short Circuiting. Separating metal from the electrolysissection removes the MHD effect of the large magnetic field on the liquidmetal pad. For example, current flow between the SOM tube(s) and thecathode does not significantly involve the metal. In conventional Hallcells, all current is between the metal and the carbon anode above,resulting in interaction with the magnetic field and waves on the metalsurface can short circuit to the carbon anode.

Simplified Feeding. Today's technology uses a frozen cryolite crust overthe top to reduce heat loss. However, feeding metal oxide raw material,e.g. alumina, requires a complex system which breaks this crust,resulting in mechanical disruption and thermal energy loss. SOMtechnology will not have a frozen crust, so illustratively a simple androbust radiation-shielded augur system with counter-current inert gasflow can feed the metal oxide into the all-liquid salt bath in awell-insulated containment system.

In one aspect, an apparatus for recovering a metal is providedcomprising: (a) a sealed container for holding a molten electrolyte, thecontainer having an interior surface; (b) a liner disposed along atleast a portion of the interior container surface; (c) a cathodedisposed to be in electrical contact with the molten electrolyte whenthe molten electrolyte is disposed in the container; (d) a solid oxygenion-conducting membrane disposed to be in ion-conducting contact withthe electrolyte when the molten electrolyte is disposed in thecontainer; (e) an anode in contact with the solid oxygen ion-conductingmembrane, the solid oxygen ion-conducting membrane electricallyseparating the anode from the molten electrolyte; and (f) a power sourcefor generating an electric potential between the anode and the cathode.Electrical separation of the anode and the molten electrolyte preventsdirect electrical contact between the anode and the molten electrolyte.

In another aspect, a method for recovering a metal is providedcomprising: (a) providing a sealed container for holding a moltenelectrolyte, the container having an interior surface; (b) providing aliner disposed along at least a portion of the interior containersurface; (c) providing a cathode disposed to be in electrical contactwith the molten electrolyte when the molten electrolyte is disposed inthe container; (d) providing a solid oxygen ion-conducting membranedisposed to be in ion-conducting contact with the electrolyte when themolten electrolyte is disposed in the container; (e) providing an anodein contact with the solid oxygen ion-conducting membrane, the solidoxygen ion-conducting membrane electrically separating the anode fromthe molten electrolyte; (f) dissolving at least a portion of an oxide ofthe metal into the electrolyte; (g) establishing a non-oxidizingenvironment within the container; and (h) generating an electricpotential between the anode and the cathode, whereby the oxide of themetal is reduced to form metal. Electrical separation of the anode andthe molten electrolyte prevents direct electrical contact between theanode and the molten electrolyte.

In some embodiments, the container comprises steel.

In some embodiments, the container is electrically isolated from thecathode.

In some embodiments, the interior surface includes a floor and the floorcomprises carbon.

In some embodiments, the liner extends from the floor upward along theinterior surface of the container. In some embodiments, the linercomprises boron nitride, Si₃N₄, fused alumina or zirconia.

In some embodiments, during generation of the electric potential, themetal is recovered from an oxide of the metal dissolved in the moltenelectrolyte, the metal collects on the floor of the container, and theliner extends to a level that prevents contact between the metal-moltenelectrolyte interface and the interior surface of the container.

In some embodiments, during generation of the electric potential, themetal is recovered from an oxide of the metal dissolved in the moltenelectrolyte and the metal collects on a top surface of the moltenelectrolyte when the electrolyte is disposed in the container, the linerextends from a first level below the metal-molten electrolyte interfaceto a second level above the metal-molten electrolyte interface, and theliner prevents contact between the metal-molten electrolyte interfaceand the interior surface of the container.

In some embodiments, a side wall of the container and the liner define apassage between the interior of the container and a well external to thecontainer.

In some embodiments, a partition is disposed inside the container, thepartition extending from a third level below the metal-moltenelectrolyte interface to a fourth level above the top surface of themetal, the third level being above a bottom surface of the container,and the partition preventing recovered metal from collecting on top of aportion of the molten electrolyte.

In some embodiments, a sheath is disposed around at least a portion ofthe solid oxygen ion-conducting membrane, the sheath extends from athird level below the metal-molten electrolyte interface to a fourthlevel above the top surface of the metal, and the sheath preventscontact between the metal being recovered and the solid oxygenion-conducting membrane. In some embodiments, the sheath comprises boronnitride, Si₃N₄, fused alumina or zirconia.

In some embodiments, the solid oxygen ion-conducting membrane andelectrical sheath define an annular space between the membrane andsheath. In some embodiments, a gas inlet in communication with theannular space is provided.

In some embodiments, the container is not electrically isolated from thecathode.

In some embodiments, the interior surface includes a floor and the floorcomprises carbon.

In some embodiments, the liner extends from the floor upward along theinterior surface of the container to a level that prevents contactbetween molten electrolyte and the interior surface of the container.

In some embodiments, a non-oxidizing environment is established. In someembodiments, an inert gas is added to the container. In someembodiments, the inert gas is dry-scrubbed with alumina after exitingthe container.

In some embodiments, at least a portion of the metal floats on themolten electrolyte.

In some embodiments, a tapping well is provided on the sealed container.

In some embodiments, a sheath is disposed around the at least a portionof the oxide ion-conducting membrane. In some embodiments, an inert gasenters the sheath-oxide ion-conducting membrane annulus. In someembodiments, the inert gas comprises a noble gas. In some embodiments,the inert gas comprises argon or nitrogen. In some embodiments, theinert gas comprises argon. In some embodiments, the inert gas comprisesnitrogen.

In some embodiments, at least a portion of the container is thermallyinsulated.

A schematic embodiment is shown in FIG. 1. An anode (100) is shown incontact with a solid oxygen ion-conducting electrolyte (105) and acurrent collector (110) that conducts electrons. The molten salt (115)is contained within an enclosed container (120) that is thermallyinsulated (125) and further contains one or a plurality of cathodes(130). Metal oxide, e.g., alumina, is fed into the container (135) andreduced to liquid aluminum metal (140), which settles toward the bottomof the container. Oxygen ions migrate from the molten salt through thesolid electrolyte to the liquid metal anode, where they form dissolvedoxygen atoms. The oxygen atoms diffuse through the liquid metal anode tothe gas phase where they form oxygen gas which flows away from the anode(See, e.g., U.S. Pat. No. 8,658,007; herein incorporated by reference inits entirety). A second type of anode (145) in contact with a secondsolid oxygen ion-conducting electrolyte (150) and fuel (e.g., naturalgas) inlet (155) can be provided in addition to or in place of anode(100). The second anode (145) enables use of less electrical energy andgeneration of water and carbon dioxide (See, e.g., U.S. PatentPublication No. 2013/0186769; herein incorporated by reference in itsentirety). An inlet (160) and outlet (165) for an inert gas such asargon is also provided.

Another schematic embodiment is shown in FIG. 2. An anode (200) is shownin contact with a solid oxygen ion-conducting electrolyte (205) and acurrent collector (210) that conducts electrons. The molten salt (215)is contained within a sealed container (220) that is thermally insulated(225) and further contains one or a plurality of cathodes (230). Metaloxide, e.g., alumina, is fed into the container (235) and reduced toliquid aluminum metal (240), which is separated from the alumina feedvia a partition (280) and removed via a tap (270) on the side of thecontainer. Oxygen ions migrate from the molten salt through the solidelectrolyte to the liquid metal anode, where they form dissolved oxygenatoms. The oxygen atoms diffuse through the liquid metal anode to thegas phase where they form oxygen gas which flows away from the anode. Asecond type of anode (245) in contact with a second solid oxygenion-conducting electrolyte (250) and fuel (e.g., natural gas) inlet(255) can be provided in addition to or in place of anode (200). Thesecond anode (245) enables use of less electrical energy and generationof water and carbon dioxide. An inlet (260) and outlet (265) for aninert gas such as argon is also provided. The inlet (260) is disposedbetween the annulus of the SOM (205) and an electrically insulatingsheath (275).

Another embodiment of a sealed (e.g. welded) steel cell (320) is shownin FIG. 3. Sealing the vessel creates a controlled environment which canbe made inert by injecting argon or helium, nitrogen, or other gas(360). This prevents rapid steel corrosion by molten salt catalysis ofsteel oxidation. It also prevents molten salt catalysis of TiB₂corrosion, enabling that cathode (330) material to protrude upward outof the salt (315). An anode (300) is shown in contact with a solidoxygen ion-conducting electrolyte (305) and a current collector (310)that conducts electrons. The molten salt (315) is contained within anenclosed container (320) that is thermally insulated (325) and furthercontains a plurality of cathodes (330). Metal oxide, e.g., alumina, isfed into the container (335) and reduced to liquid aluminum metal (340),which settles toward the bottom of the container. Oxygen ions migratefrom the molten salt through the solid electrolyte to the liquid metalanode, where they form dissolved oxygen atoms. The oxygen atoms diffusethrough the liquid metal anode to the gas phase where they form oxygengas which flows away from the anode. A second type of anode (345) incontact with a second solid oxygen ion-conducting electrolyte (350) andfuel (e.g., natural gas) inlet (355) can be provided in addition to orin place of anode (300). The second anode (345) enables use of lesselectrical energy and generation of water and carbon dioxide. An inlet(360) and outlet (365) for an inert gas such as argon is also provided.

In some embodiments, the injection site for the inert gas is submergedin the bath, in order to create gas lift stirring and promote oxidecirculation. The cathode material is advantageously compatible withliquid metal and molten salt and has high electronic conductivity. Insome embodiments, the cathode comprises TiB₂. To reduce the cost of TiB₂for cathodes, closed-end TiB₂ tubes can be used, and filled with ahigh-conductivity liquid metal such as aluminum, copper, tin or bismuth(See, e.g., U.S. Pat. No. 4,612,103; herein incorporated by reference inits entirety).

The liquid aluminum product (340), which can act as a cathode, rests ona low-cost carbon floor (385) to prevent it from contacting the steel(320). At the aluminum-salt bath interface level, a liner material (390)prevents aluminum reaction with the steel vessel, and also saltcatalysis of aluminum carbide (Al₄C₃) formation. Because thealuminum-salt bath interface level rises and falls with production andwithdrawal of liquid aluminum, the liner extends vertically over thefull extent of the rising and falling of the interface. In someembodiments, it is advantageous that the liner extend over the entirevertical interior surface of the vessel.

The liner can be one of several materials stable in contact with steel,not soluble in liquid metal, and either not soluble or slow-dissolvingin the molten salt bath. Exemplary materials include stabilizedzirconia, such as yttria-stabilized zirconia similar to that used in thezirconia tubes, or low-cost calcia-stabilized zirconia; oxide materialssuch as alumina, e.g. fuse-cast alumina; other compounds such as boronnitride, TiB₂, SiC, and/or Si₃N₄.

The liner materials can be applied by several methods including:insertion of plates, tiles, or bricks; or thermal spray to create anadhering layer of material on the steel and carbon. The liners need notcreate a perfect seal between the steel vessel and liquid metal ormolten salt bath. The inert environment makes the steel vessel stable inthe molten salt, and the liner need only slow down metal-steelinteraction kinetics sufficiently to prevent liquid metal from breakingout of the steel vessel, and to prevent steel from contaminating theliquid metal beyond the product composition specification.

The steel vessel creates opportunities for thermal manipulation. Forexample one can apply heating (including fuel-fired heaters) directly tothe steel exterior during start-up or accidental cooling due to powerfailure. If the cell gets too hot, removal of insulation or blowingcooling fluid through insulation can help to cool it. This canpotentially reduce or eliminate thermal runaway problems during avoltage/temperature increase event caused by metal oxide (e.g., alumina)depletion.

Traditional cells such as Hall-Héroult cells do not have this advantage.Cell heat loss through the side walls is fixed by the need to maintainprecise crust geometry. If too much heat is extracted, the crust grows,freezing the bath from the sides and reducing current until the cellshuts down. If too little heat is extracted, the crust shrinks, leadingto liquid bath-metal interface contact with carbon lining and rapiddissolution of the carbon. In the instant invention, the cell canoperate over a much wider range of temperature and heat flux withoutcompromising containment or hindering cell operation.

In addition, in some embodiments the apparatus and method provides oneor more of the following benefits: high productivity per unitcross-section area due to the high surface area of vertical cathodes andzirconia-encased anodes; insulation of steel to reduce vessel thermalloss well below that of a conventional cell; opportunity for directexternal heating or cooling of the steel vessel; simplified feeding dueto lack of frozen salt crust; in situ dry scrubbing of the inert gas(illustratively argon, but could also be other gases) by counter-currentflow of argon out through the metal oxide feeder.

Another embodiment of a sealed cell (420), high-density salt bath isshown in FIG. 4, with the electrolysis cell configuration with ahigh-density molten salt (415) bath, i.e. denser than liquid metal(e.g., aluminum) (440) at the cell operating temperature. It also uses aliner (490) from the same candidate material list and application methodto prevent aluminum-steel contact and to prevent molten salt catalysisof aluminum carbide (Al₄C₃) dissolution. This configuration also uses agas to create an inert or reducing environment and prevent corrosion ofthe steel vessel in the molten salt. In some embodiments, the injectionsite for the inert or reducing gas is submerged in the bath, in order tocreate gas lift stirring and promote oxide circulation. An anode (400)is shown disposed in contact with a solid oxygen ion-conductingelectrolyte (405) and a current collector (410) that conducts electrons.The molten salt (415) is contained within a sealed container (420) thatis thermally insulated (425) and further contains a plurality ofcathodes (430). Metal oxide, e.g., alumina, is fed into the container(435) and reduced to liquid aluminum metal (440), which is separatedfrom the alumina feed via a partition (480) and removed via a tap (470)on the side of the container. Oxygen ions migrate from the molten saltthrough the solid electrolyte to the liquid metal anode, where they formdissolved oxygen atoms. The oxygen atoms diffuse through the liquidmetal anode to the gas phase where they form oxygen gas which flows awayfrom the anode. A second type of anode (445) in contact with a secondsolid oxygen ion-conducting electrolyte (450) and fuel (e.g., naturalgas) inlet (455) can be provided in addition to or in place of anode(400). The second anode (445) enables use of less electrical energygeneration of water and carbon dioxide. An inlet (460) and outlet (465)for an inert gas such as argon is also provided. The inlet (460) isdisposed between the annulus of the SOM (405) and an electricallyinsulating sheath (475).

If the floating liquid metal is connected to the cathodes, then thisconfiguration requires sheathing around zirconia tubes to separate theliquid metal (e.g., aluminum) from the tubes, in order to preventshort-circuiting and damage to the zirconia. In some embodiments, inertor reducing gas bleeding into the sheath-zirconia annulus can preventmolten electrolyte or metal from entering that annulus. Thus, theannular region allows for a non-electrical insulator to be used as thesheath. That sheath can be made of one of the materials in the linerlist, though an electronic insulator such as zirconia, alumina or boronnitride is preferred for preventing shortcircuiting rather than aconductor such as TiB₂. That sheath can be a separate tubular partsurrounding the zirconia tube, or it can be a coating attached to thetube. In some embodiments, insulating sheaths are placed around the SOMand inert gas inlet into the zirconia-sheath annulus preventsshort-circuiting in the floating metal configuration. The sheath andinert gas block electrical connection between the cathode and thezirconia, and reduction of the zirconia. The sheath can be boronnitride, silicon nitride, zirconia with low ionic conductivity. Thesheath can also be ion-conducting zirconia or electron-conducting metal;however if the sheath is conducting, it is advantageous that the SOMdoes not contact the sheath. Thus, non-conducting spacers can beinserted between the sheath and the SOM.

In some embodiments, this configuration uses a dam, tube, or similarpartition to create an opening through the floating liquid metal andpermit direct feeding of metal oxide into the molten salt bath. Thisconstraint material preferably has minimal reaction with the metal andmolten salt, such that zirconia, TiB₂, boron nitride, or similarmaterials will have long lifetime in this function.

In some embodiments, larger metal oxide (e.g., alumina) pellets are fedinto the liquid metal (e.g., aluminum), when the metal oxide has higherdensity than liquid metal, such that as long as surface tension does notsupport the metal oxide, it will sink through the metal into the saltand dissolve therein.

Floating metal configurations enable simplified tapping using a separatechamber, or seal pot, holding liquid metal, which is lined with carbon,boron nitride, TiB₂ or similar material not soluble in the metal. Anopening in the vessel can allow liquid metal to flow out of the vesseland into the seal pot.

In some embodiments, the floating metal configuration adds greaterflexibility in metal oxide feed morphology: pellets of sintered metaloxide fines which fall to the bottom slowly dissolve in the molten salt,where in the other configurations or the conventional Hall-Héroult cellthey would sink through the liquid metal and accumulate as sludge; lesscontamination of metal product if a zirconia tube breaks and releasesthe liquid metal anode, which will likely have higher density than thesalt bath as the main candidates are silver, copper and tin; andopportunity for simplified tapping using the liquid metal seal pot withtop tapping.

A sealed container, e.g. steel, can be used to produce metal and isenabled by creation of a non-oxidizing environment such as a reducing orinert environment. The sealed container prevents ingress of air and/orgas that would react with the liquid metal or corrode the interior ofthe container. The non-oxidizing environment prevents or minimizesoxidation of the steel container catalyzed by the molten electrolyte.The non-oxidizing environment exists because oxygen containingcomponents (oxygen gas, carbon dioxide or water vapor) are positioned onthe inside of the zirconia tube, and the applied electrical potentialdrives oxygen into the zirconia tube from the molten salt outside of it.Such configurations enable insulation to reduce thermal energy lossand/or allow the process to run at high salt superheat (above itsmelting point), which avoids the complication of crust management inconventional systems. Thus, feeding metal oxide is simplified becausethere is no crust to break in order to mix the metal oxide into themolten electrolyte. Embodiments where liquid metal is disposed on top ofthe molten electrolyte salt are also enabled by the non-oxidizingenvironment, as the non-oxidizing environment prevents oxidation of thefloating metal.

A non-oxidizing environment can be established by any number of waysincluding, e.g., providing an inert gas and, optionally,electrochemically pumping oxygen by establishing an anode-cathodepotential, or by bubbling inert gas below the molten electrolyte and/orthe liquid metal. Bubbling inert gas below the motel electrolyte and/orliquid metal advantageously provides mixing to assist the metal oxidereach the oxide-ion conducting membrane.

The inert gas may comprise a noble gas, e.g., argon, or nitrogen. Otherinert gases include SF₆ and CF₄. In some embodiments, the inert gascomprises a noble gas or nitrogen. In some embodiments, the inert gascomprises argon or nitrogen. In some embodiments, the inert gascomprises argon. In some embodiments, the inert gas comprises nitrogen.

Dry scrubbing the inert gas is also an optional process. Thus, any HFpresent (e.g., in the inert gas or otherwise) can be absorbed by themetal oxide, e.g., alumina, and converted to metal fluoride (e.g.,aluminum trifluoride) and water. In some embodiments, the inert gas exitbleed (265) is controlled and carries HF to the metal oxide (235) toform metal fluoride. By feeding the resulting metal fluoride back intothe molten electrolyte, such embodiments preserve fluoride contents inthe system. Metal oxide particles known to avoid sludge problems canalso be used for the scrubbing process.

In some embodiments wherein metal is disposed on top of the moltenelectrolyte salt, a side tapping well is added on the container. Thewell enables removal of metal from the apparatus. In some embodimentswherein metal is disposed on top of the molten electrolyte salt, apartition or dam is provided to separate the metal from a portion of thetop surface of the molten salt electrolyte, such that the metal oxidecan be fed directly into the salt.

The metal produced is not limited to aluminum. In some embodiments, themetal is any metal that has a density less than the molten saltelectrolyte at the operating temperature of the cell. Thus, in someembodiments, the metal will float on top of the electrolyte. In someembodiments, the metal comprises aluminum, magnesium, lithium,beryllium, silicon, sodium, potassium or calcium. In some embodiments,the metal comprises aluminum, lithium, beryllium, silicon, sodium, orpotassium. In some embodiments, the metal comprises aluminum, lithium,beryllium, sodium, or potassium. In some embodiments, the metalcomprises aluminum, lithium, sodium, or potassium. In some embodiments,the metal comprises aluminum. It is understood that the metal oxidecomprises an oxide of the metal to be produced.

In some embodiments employing a fueled anode, the fuel comprisesmethane, syngas, hydrogen, or other hydrocarbons. In some embodiments,the fuel tube comprises a conductive metal, such as nickel, molybdenumor cobalt, and can be attached to, and form part of, the currentcollector. In some embodiments, the fuel delivery tube is a stableoxide, such as aluminum oxide, mullite, or magnesium oxide, such that itis stable in both oxygen and fuel gas, and the device can operate ineither oxygen production or fueled modes depending on the flow rate offuel.

In some embodiments, the solid electrolyte comprises zirconia doped withyttria, calcia, magnesia, scandia, other rare earth oxide, or otheradditives that stabilize its cubic phase and enhance its conductivity;or ceria doped with oxides to increase its ion, e.g oxygen,conductivity; or any other oxygen ion-conducting solid electrolyte. Insome embodiments, the solid electrolyte comprises zirconia doped withyttria, calcia, magnesia, scandia, or other rare earth oxide; or ceriadoped with oxides to increase its oxygen ion conductivity. In someembodiments, the solid electrolyte comprises zirconia doped with yttria,calcia, magnesia, scandia, or other rare earth oxide. In someembodiments, the solid electrolyte comprises zirconia doped with yttria,calcia, magnesia, or scandia. In some embodiments, the solid electrolytecomprises ceria doped with oxides.

The molten electrolyte composition may be comprised of severalcomponents. Preferred molten electrolyte systems are selected based onseveral criteria:

Cation oxide free energy. All salt cation species ideally have oxidefree energies of formation more negative than that of the metal, suchthat the process does not reduce spectator cations along with theproduct. Representative cation species include magnesium, sodium,cerium, lanthanum, calcium, strontium, barium, lithium, potassium andytterbium. Thus, in some embodiments, the molten electrolyte comprisescations of magnesium, sodium, cerium, lanthanum, calcium, strontium,barium, lithium, potassium or ytterbium. In some embodiments, the moltenelectrolyte comprises cations of magnesium, sodium, cerium, andlanthanum.

Low volatility. Preferably the salt exhibits very low vapor pressure andevaporation rate in the process temperature range. Combiningthermo-gravimetric analysis (TGA) with differential scanning calorimetry(DSC) or differential thermal analysis (DTA) experiments can efficientlyevaluate the salt for this criterion. In some embodiments, fluoridesalts are preferable over chlorides, and the volatility of lithiumfluoride makes it less attractive.

Low melting point. A preferred range of about 900-1200° C. providesbalance between energy efficiency and material flexibility at lowtemperature, and good oxide ion conductivity in stabilized zirconia athigh temperature. The salt must be liquid at the operating temperature.

High ionic conductivity supports high current density withoutsignificant transport limitation. By way of example, a salt with lowionic conductivity inhibits mass transfer to the zirconia and thecathode; at the zirconia oxygen ions are depleted in the boundary layer,reducing the current, and at the cathode the target metal ions aredepleted in the boundary layer, leading to reduction and co-depositionof salt cations, reducing purity.

Low viscosity. Viscosity inhibits mass transport to electrodes. Saltswith high fluoride/oxide ratio have had sufficiently high ionicconductivity and low viscosity to support up to about 2 A/cm² anode andcathode current density.

Target oxide solubility. The salt must dissolve the metal oxide to atleast about 3-5 wt % in order to achieve preferred ionic currentdensities at the anode and cathode. DSC or DTA experiments at variouscompositions can efficiently characterize oxide solubility.

Low metal solubility and electronic conductivity. If the salt is a goodelectronic conductor, then it effectively becomes an extended cathodeand can reduce the zirconia. This is impacted in part by solubility ofthe reduced metal in the salt, e.g. calcium metal is soluble in manysalts. Methods for reducing metal solubility inline are alsocontemplated (See, e.g., U.S. Patent Publication No. 2013/0152734;herein incorporated by reference in its entirety).

Zirconia stability. Salt corrosion of the zirconia solid electrolyte ispreferably very slow. Ideally, the salt preferably satisfies twocriteria: salt optical basicity and yttria (or other stabilizing oxide)chemical potential are both close to those values in the zirconia. Toevaluate stability, zirconia is immersed in the salt at the processtemperature for several hundred hours, then sectioned and characterized.

Fluoride salts are particularly preferred as they offer advantageouscombinations of properties due to their low volatility and viscosity,high oxide solubility and ionic conductivity, and low basicity.

Preferred molten salts comprise LiF, NaF, CaF₂, MgF₂, SrF₂, and AlF₃.These have melting points around 700° C. with relatively low vaporpressure. The optimal molten salt will have maximum metal oxidesolubility and minimum metal solubility.

In some embodiments, a liner is disposed between the container and themetal. Exemplary materials for the liner include carbon, boron nitride,titanium diboride, SiC, Si₃N₄, fused alumina or zirconia.

In some embodiments, a second liner is disposed along at least a portionof an interior surface of the container. In some embodiments, linerprevents contact between the container and the molten salt electrolyte.The liner and/or insulating sheath may be applied, e.g., as apre-fabricated sheath or sheath components, including tiles or bricks,or via thermal spraying onto the desired surface such as plasmaspraying.

The anode may be an oxygen-generating inert anode, or a fueled anode. Insome embodiments, the fuel comprises natural gas. In some embodiments,liquid silver anodes are preferred. Porous perovskite conductors ofsolid oxide fuel cell (SOFC) technology, such as La 0.8 Sr 0.2 MnO₃(LSM), are potential candidates for this role. Other potential anodematerials comprise antimony, bismuth, copper, gallium, indium, and tin.

It will be recognized that one or more features of any embodimentsdisclosed herein may be combined and/or rearranged within the scope ofthe invention to produce further embodiments that are also within thescope of the invention.

Those skilled in the art will recognize, or be able to ascertain usingno more than routine experimentation, many equivalents to the specificembodiments of the invention described herein. Such equivalents are alsointended to be within the scope of the present invention.

EXAMPLES

The examples provided below facilitate a more complete understanding ofthe invention. The following examples illustrate exemplary modes ofmaking and practicing the invention. However, the scope of the inventionis not limited to specific embodiments disclosed in such examples, whichare illustrative only, since alternative methods can be utilized toobtain similar results.

A salt comprising aluminum fluoride and aluminum oxide was placed into astainless steel crucible and heated in an argon environment to 105° C.,then 400° C., and finally to 1000° C. While at temperature, the salt wasstirred using a stainless steel mixer to ensure homogeneity and expeditethe dissolution of aluminum oxide. The crucible and salt were thencooled and removed from the furnace.

After cooling, a stainless steel block containing two boron nitride cups(one to insulate the anode and another to collect aluminum metal) wasplaced into the crucible on top of the salt block in the crucible. Athreaded rod was inserted into the block in order to facilitate theraising and lowering of the block. A YSZ SOM, zirconium diboride (ZrB₂)cathode, platinum reference electrode, and a thermocouple were alsoinserted into the crucible. A silver anode and an oxygen-stable currentcollector (following the design in U.S. patent application Ser. No.13/600,761 publication 2013/0192998; herein incorporated by reference inits entirety) were inserted inside the YSZ SOM. Both the cathode andreference electrode were attached to stainless steel rods with boronnitride sheaths in between in order to insulate them at the salt/argoninterface.

The cage was placed in the furnace and heated to 970° C. After the saltwas molten, the stainless steel block was lowered to the bottom of thecrucible. The electrodes were lowered simultaneously; the anode into oneboron nitride cup in the stainless steel block, the cathode to rightabove the other boron nitride cup, and the reference electrode to justunder the salt surface. The current collector was lowered into theliquid silver anode.

At this point AC impedance measurements were taken between theelectrodes, and chronoamperaometry measurements were made at severalvoltages below the reduction potential of aluminum. Following thesemeasurements electrolysis was started at 5 volts for 203 minutesproducing 2.78 liters of oxygen gas at the anode.

Upon cooling to room temperature, the aluminum metal product wasrecovered from the top surface of the salt attached to the boron nitridesheath on the ZrB₂ cathode.

This experiment demonstrated the ability of the non-oxidizingenvironment to allow production of aluminum on the top surface of thesalt, without oxidizing as it would have in the presence of air or otheroxidizing gas.

As will be apparent to one of ordinary skill in the art from a readingof this disclosure, further embodiments of the present invention can bepresented in forms other than those specifically disclosed above. Theparticular embodiments described above are, therefore, to be consideredas illustrative and not restrictive. Those skilled in the art willrecognize, or be able to ascertain, using no more than routineexperimentation, numerous equivalents to the specific embodimentsdescribed herein. Although the invention has been described andillustrated in the foregoing illustrative embodiments, it is understoodthat the present disclosure has been made only by way of example, andthat numerous changes in the details of implementation of the inventioncan be made without departing from the spirit and scope of theinvention, which is limited only by the claims that follow. Features ofthe disclosed embodiments can be combined and rearranged in various wayswithin the scope and spirit of the invention. The scope of the inventionis as set forth in the appended claims and equivalents thereof, ratherthan being limited to the examples contained in the foregoingdescription.

1.-17. (canceled)
 18. A method for recovering a metal, comprising: (a)providing a sealed container for holding a molten electrolyte, thecontainer having an interior surface; (b) providing a liner disposedalong at least a portion of the interior container surface; (c)providing a cathode disposed to be in electrical contact with the moltenelectrolyte when the molten electrolyte is disposed in the container;(d) providing a solid oxygen ion-conducting membrane disposed to be inion-conducting contact with the electrolyte when the molten electrolyteis disposed in the container; (e) providing an anode in contact with thesolid oxygen ion-conducting membrane, the solid oxygen ion-conductingmembrane electrically separating the anode from the molten electrolyte;(f) dissolving at least a portion of an oxide of the metal into theelectrolyte; (g) establishing a non-oxidizing environment within thecontainer; and (h) generating an electric potential between the anodeand the cathode, whereby the oxide of the metal is reduced to formmetal.
 19. The method of claim 18, wherein the container comprisessteel.
 20. The method of claim 18, wherein the container is electricallyisolated from the cathode.
 21. The method of claim 18, wherein theinterior surface includes a floor and the floor comprises carbon. 22.The method of claim 21, wherein the liner extends from the floor upwardalong the interior surface of the container.
 23. The method of claim 22,wherein, during generation of the electric potential, the metal isrecovered from metal oxide dissolved in the molten electrolyte and themetal collects on the floor of the container and wherein the linerextends to a level that prevents contact between the metal beingrecovered and the interior surface of the container.
 24. The method ofclaim 18, wherein the liner comprises carbon, boron nitride, titaniumdiboride, SiC, Si₃N₄, fused alumina or zirconia.
 25. The method of claim18, wherein, during generation of the electric potential, the metal isrecovered from a metal oxide dissolved in the molten electrolyte and themetal collects on a top surface of the molten electrolyte when theelectrolyte is disposed in the container and wherein the liner extendsfrom a first level below the metal-molten electrolyte interface to asecond level above the metal-molten electrolyte interface, and the linerprevents contact between the metal-molten electrolyte interface and theinterior surface of the container.
 26. The method of claim 25, wherein aside wall of the container and the liner define a passage between theinterior of the container and a well external to the container.
 27. Themethod of claim 26, further comprising providing a partition disposedinside the container, the partition extending from a third level belowthe metal-molten electrolyte interface to a fourth level above the topsurface of the metal, the third level being above a bottom surface ofthe container, and the partition preventing recovered metal fromcollecting on top of a portion of the molten electrolyte.
 28. The methodof claim 18, further comprising providing a sheath disposed around atleast a portion of the solid oxygen ion-conducting membrane, the sheathextending from a third level below the metal-molten electrolyteinterface to a fourth level above the top surface of the metal, and thesheath preventing contact between the metal being recovered and thesolid oxygen ion-conducting membrane.
 29. The method of claim 28,wherein the sheath comprises boron nitride, Si₃N₄, fused alumina orzirconia.
 30. The method of claim 28, the solid oxygen ion-conductingmembrane and sheath defining an annular space between the membrane andsheath, the method further comprising providing a gas inlet incommunication with the annular space.
 31. The method of claim 18,wherein the container is not electrically isolated from the cathode. 32.The method of claim 31, wherein the interior surface includes a floorand the floor comprises carbon.
 33. The method of claim 33 claim 32,wherein the liner extends from the floor upward along the interiorsurface of the container to a level that prevents contact between moltenelectrolyte and the interior surface of the container.
 34. The method ofclaim 311 claim 33, wherein the liner comprises boron nitride, Si₃N₄,fused alumina or zirconia.
 35. The method of claim 18, wherein the metaloxide is fed directly into the molten electrolyte.
 36. The method ofclaim 18, wherein the metal comprises aluminum, magnesium, lithium,beryllium, silicon, sodium, potassium or calcium.
 37. The method ofclaim 36, wherein the metal comprises aluminum, lithium, beryllium,silicon, sodium, or potassium.
 38. The method of claim 37, wherein themetal comprises aluminum.